In line with rapid increase in use of fossil fuels, demand for alternative energy or clean energy is increasing. Thus, the field of power generation and electrochemical electricity storage is most actively studied.
As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and use thereof is gradually expanding.
Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating potential, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles, hybrid electric vehicles, and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway, and lithium secondary batteries are being used as a power source of electric vehicles, hybrid electric vehicles, and the like.
In general, lithium secondary batteries are charged and discharged through a process wherein lithium ions of a positive electrode are repeatedly intercalated and deintercalated at a negative electrode. As such lithium ions are repeatedly intercalated and deintercalated, a bond between an electrode active material or a conductive material becomes loose and contact resistance between particles increases. As a result, ohmic resistance of an electrode increases and thus battery properties may be deteriorated. Accordingly, since a binder should perform buffer action for expansion and contraction of an electrode active material due to intercalation and deintercalation of lithium ions in an electrode, a polymer having elasticity is preferable.
In addition, a binder should have adhesion such that binding capacity between an electrode active material and a current collector is maintained in a process of drying a plate. In particular, when natural graphite having a theoretical discharge capacity of 372 mAh/g is used with a material such as silicon, tin or silicon-tin alloy having high discharge capacity so as to increase discharge capacity, volume expansion dramatically decreases as charge and discharge proceed and thus a negative electrode material is desorbed. As a result, battery capacity may be dramatically decreased with increasing number of charge and discharge cycles.
Furthermore, since electrolyte swelling of a binder affects volume expansion of lithium ion batteries, a binder having low electrolyte swelling is required. Lithium ion batteries swell due to gas generated when an electrolyte inside the batteries is decomposed. Such a phenomenon is called electrolyte swelling. Since decomposition of electrolytes is accelerated at high temperature, swelling increases when batteries are neglected. When temperature of batteries is elevated, electrolyte is decomposed or side-reaction occurs. Accordingly, gas such as carbon dioxide or carbon monoxide is generated and thus the thicknesses of batteries increase. Meanwhile, thickness increase during high-temperature storage is an important consideration in small batteries. Temperatures of small batteries are rapidly elevated during use thereof, and thus, there are problems in thickness and stability at high temperature.
Thus, there is an urgent need in the art to study a binder and an electrode material that may have strong adhesive strength so as to prevent separation between electrode active material components or separation between an electrode active material and a current collector when manufacturing an electrode and may have strong physical properties so as to achieve structural stability of an electrode by controlling volume expansion of an electrode active material due to repeated charge/discharge and, accordingly, enhance battery performance.
As currently, commercially available representative binders, there are polyvinylidene fluoride (PVdF), styrene-butadiene rubber (SBR)/carboxy methyl cellulose (CMC), etc. In the case of negative electrodes having great volume expansion during charge/discharge when compared with positive electrodes, SBR/CMC that is used in a smaller amount than PVdF and has superior binding capacity is used.
A conventional solvent-based binder, i.e., polyvinylidene fluoride (PVdF), does not meet such requirements and thus, recently, a method of using binders prepared by preparing emulsifier particles by aqueous polymerization of styrene-butadiene rubber (SBR) and mixing the emulsifier particles with a neutralizing agent and the like has been proposed and is currently commercially available. These binders are eco-friendly and are used in small amounts, thus increasing battery capacity. However, such a case also exhibits improved adhesive durability due to rubber elasticity but does not exhibit dramatically improved adhesive strength.
Therefore, there is an urgent need to develop a binder that enhances cycle characteristics of a battery, imparts structural stability to an electrode, and has high adhesive strength.